The City of Copenhagen recently launched a public bicycle counter [copenhagenize.com], completely equipped with an air pump for the convenience of cyclists. The urban display counts the daily number of cyclists that use the new Green Path that slices diagonally across the Copenhagen and Frederiksberg pathway system. There is a 'sensor line' in the asphalt on the bike lane a few metres in front of the counter which registers the cyclists, probably via a motion sensor.

The idea is to encourage more people to ride by showing how many are using it. The numeric displays show the total so far today and this year. On the barometer-styled display, the left side will show last years' total.

Probably an induction loop still. Motion sensor (by which I assume you mean something optical, acoustical or RF in free air) will pick up walkers also. And dogs and possums and tumbleweeds. Induction can detect metal, and even the most snazzy of lightweight CFRP bikes have more metal than most people and much closer to the ground.
http://www.humantransport.org/bicycledriving/library/signals/detection.htm
While most streets are not set up right still, it's possible to do this right.

I Like the idea of people travelling around on bikes. If nothing else it encourages people to keep fit. Cambridge could do with one of these counters, I am sure the figures would make fascinating reading.

Stockholm has had a similar counter close to Slussen for a couple of years. It may be gone now with all the building activity going on there. That instance counts on a day to day-basis, IIRC.

As a bicycle rider, the lower thing to the left with some tools, tissue paper and high pressure air ("LUFT", for tires) is more useful but if you pass it on your way every day it is nice to see the days count.